“New and Improved”: The Crucial Importance of Unlinking “Different” from “Higher and Lower”
As with the excretory system example cited above, when I contrast the incompletely divided heart of amphibians and reptiles with the fully divided four-chambered heart of birds and mammals, I am careful not to refer to the former condition as an inferior forerunner of the latter. As I make clear, frogs are perfectly happy being frogs, thank you—they are not trying to ascend an evolutionary ladder on a trek toward humanity (Mead 2009). Amphibians and reptiles have been successful on their own as measured by diversity, abundance, and evolutionary time span. If they were not, they would not have survived.
Indeed, the incomplete division of the ventricle can be seen as a useful adaptation given that many amphibians and reptiles undergo prolonged periods of apnea (breath-holding), in which a circulatory shunt bypassing the lungs and diverting blood instead to the systemic circulation is highly effective (Hill et al. 2008). The more active endothermic birds and mammals, with their high metabolic rates, have no use for such an adaptation. In other words, the incompletely divided heart can, and probably should, be seen as advantageous rather than limiting. To amphibians and reptiles, it is a benefit, not a drawback. Why is it seldom presented this way? It is because the notion of inevitably increasing progress (Ruse 1997; Carroll 2001) is a difficult snare to avoid, even for trained biologists. When viewed from the human perspective, it is easy to fall into the trap of equating “different” with “inferior.” No less an authority than Darwin himself admonished, “Never use the words higher and lower” (Desmond and Moore 1994). We expect that all change is for the better and that any derived taxon or condition is not only new but “improved” from the “primitive” (ancestral) condition. Natural selection produces highly complex structures (Petto and Mead 2008), but evolution sometimes leads to less complexity (e.g., mammalian skulls have fewer bony elements and moving parts than the reptilian skulls they evolved from). As with the cases of excretory systems and hearts, different does not imply better or worse. Consider, as another example, the fact that humans, like other vertebrates, raise cardiac output by increasing heart rate, whereas invertebrates tend to boost cardiac output by increasing stroke volume instead. Both solutions achieve the desired outcome, though in entirely different ways. Can we say that one solution is better?
Living Fossils and Dead Fossils: Does Paleontology Support the Notion of Increasing Complexity?
McShea (1991, 1996, 2001) claims that, although the idea of rising complexity throughout evolutionary history is widespread and deeply entrenched, the empirical evidence for this claim is spotty and depends on the eye of the beholder (Gregory 2008a; Petto and Mead 2008). For example, the evolution of some segmented animal phyla involved an apparent loss or fusion of various body segments into a smaller number of simpler and stronger if more diverse segments. An example repeatedly cited by McShea (1991, 1993) involves the evolution of the vertebrate spine: A survey of mammalian lineages shows an equal number of increases and decreases in complexity. The origin of the tetrapod vertebral column (holospondylous and monospondylous, meaning with a single fused centrum per body segment) from that of ancestral fishes likewise demonstrates a marked decrease in vertebral complexity. With a little effort, one can easily think of other examples where organisms have evolved to be less complex, such as endoparasites losing organ systems, avian lineages leading to flightless species, or snakes, whales, and other tetrapods losing limbs. In the case of aquatic ancestors leading to terrestrial descendants that later returned to the sea, this involved an abrupt about-face or reversal of “progress.”
Counterintuitively, “higher” organisms do not necessarily have larger genomes than “lower” taxa. Humans have fewer genes than many plant species and fewer base pairs than many plants, invertebrates, fishes, amphibians, and protists (Raven et al. 2007). A decade ago, biologists presumed that the human genome included perhaps a quarter of a million genes, but the current best estimate involves a tenfold decrease (to 25,000 or so), though we now understand that with alternative gene splicing, a smaller number of genes can produce large numbers of polypeptides and highly complex phenotypes.
Still, McShea and Brandon argue (2010) that “in any evolutionary system where there is variation and heredity, there is, in the absence of constraint, a tendency for diversity and complexity to increase.” Such constraints may be intrinsic (e.g., genetic) or extrinsic (environmental), and this may be why some “living fossils,” including horseshoe crabs and horsetail (Equisetum) plants, have scarcely changed if at all over hundreds of millions of years. Countless other examples of apparent evolutionary stasis (at least in morphology) can be cited, ranging from brachiopods, lungfish, lampreys, coelacanths, and the chambered nautilus to lycopods, cycads, tree ferns, and other plants including the ginkgo, dawn redwood, and Wollemi pine. It is useful to remind students of Van Valen’s “Red Queen” hypothesis (1973), which holds that—like the Queen of Hearts in Alice in Wonderland, who claimed that “It takes all the running you can do, to keep in the same place”—organisms exist in complex ecological webs and often evolve mainly to keep up with constant changes in the biotic or abiotic parts of their ecosystems. It may well be that “living fossils” have evolved greatly in biochemistry, physiology, or behavior (or just in non-coding DNA, with the ceaseless ticking of the molecular clock), even if such changes are not apparent in divergence of form from specimens in the fossil record.
King of the Hill: Can Some Species be “More Evolved”?
Students often speak of species that are “more evolved.” It is hard to know exactly what this means. Are not all extant organisms equally evolved? My guess is that even while accepting biological evolution, people cannot break free from the tyranny of Aristotle’s “Great Chain of Being,” with humans enthroned as “King of the Hill.” We forget that, since the lineages of amphibians and amniotes diverged, both groups have had the same time period over which to change. In other words, they are equally distant from their most recent common ancestor. As Dawkins (2004) warns, we must be cautious when we say, for instance, that onychophoran velvet worms (Peripatus) “bridge the gap” between annelids and arthropods. What gap? If we think of evolution in terms of a progressive chain or ladder, it is easy to envision missing links or steps. But this is surely an unrealistic view (Schwenk 2002). We imagine all fossil forms as ancestors on a direct, progressive path. We forget that The Origin of Species has but one diagram, signifying evolutionary relationships: not a ladder or chain, but a bush. In this regard, it is instructive to ask students to explain in what ways Homo sapiens is more evolved than Escherichia coli. Yes, bacteria more closely resemble the last universal common ancestor, but does this mean they have stopped evolving?
Don’t Adaptive “Arms Races” Mean that Evolutionary Change Always Reflects “Improvement”?
This is not to imply that the issue of progress in evolution is simple. Although Stephen Jay Gould (1989) vehemently cautioned against equating change with progress, Darwin’s bulldog himself (Thomas Henry Huxley) and his grandson Julian Huxley both argued that evolutionary advances could be considered tantamount to progress (Desmond 1997). A book by philosopher Michael Ruse (1997) is subtitled “The Concept of Progress in Evolutionary Biology,” attesting to the prevalence of such arguments and predominance of this view. One cannot deny that strong selective pressures push gene pools (as in directional selection) quite steadily, often in a ratcheting process, or that coevolutionary arms races lead to spiraling increases in adaptations. Adapting to different peaks on an adaptive landscape can lead to some evolutionary outcomes being more likely than others, as in convergent evolution. However, we must be careful not to equate newer with better, and we must be especially vigilant in not confirming students’ default expectations that evolutionary change is a fulfillment of some grand plan. Alas, the very word “evolution” itself refers to unrolling or unfolding, as of a previously penned script. We must make clear that evolution operates in the present, without looking ahead or planning for a preordained future.
Function vs. Purpose: The Pervasive Peril of Teleological Thinking
The seductive siren of teleology has long been recognized (Ayala 1970; Jungwirth 1975; Bartov 1978; Lennox 1993; Dawkins 1995; Williams 1998; Ruse 2000b; Zohar and Ginossar 1998; Zeigler 2008; González Galli and Meinardi 2010; Mead and Scott 2010a, b), but recent studies (Kelemen and Rosset 2009) reveal just how automatic and deep-seated our tendency is to explain natural phenomena in terms of purpose and how this contributes to our view of evolution as progressive (Quammen 2006). We are intentional creatures; we see intention everywhere (Kelemen 2003). Although research reveals that this inclination is independent of religious proclivity (Kelemen 2004a, b), the propensity for teleology provides natural support to creationist tendencies (Atran 1998; Kelemen 1999; Kelemen and Rosset 2009), making the job of biology teachers all the more difficult (and imperative). Paley’s argument from design remains a major obstacle to evolutionary thinking (Gregory 2009a; Mead and Scott 2010a). As Gregory (2009b) warns, teleological explanations are so deeply ingrained that even biologists say such things as “fungi grow in forests to help decomposition” or “finches diversified in order to survive” (Kelemen and Rosset 2009).
When I explain to students that, strictly speaking, a heart or wing has no purpose, they invariably look at me cross-eyed. When I go on to explain that these structures indeed have functions, but that the word “purpose” implies, as in Aristotle’s telos (final cause), a foreseen goal or end point, they finally see the crucial distinction, which is that other, equally effective solutions could have evolved to pump blood or generate lift forces for flight. The difference in semantics may be negligible, but in viewpoint it is great.
Wings and Flippers: Doesn’t Convergence Indicate Evolutionary Progress?
The fact that many similar features appear throughout evolution gives the impression of inexorable evolutionary progress. Does not the way that marine mammals (cetaceans, sirenians, pinnipeds) have independently evolved flippers lend credence to the notion that common solutions are inevitable? Kelly (2010) argues that the repeated evolution of such traits as flapping flight, echolocation, camera-like eyes, brains, and even intelligence was not accidental but represents an inevitable outcome. Conway Morris (2003) further argues that catastrophic events such as asteroid impacts or other major extinction events can delay evolutionary events but ultimately affect only the timing, not the eventual, inexorable result. In fact, the observation that similar features arise again and again via convergent evolution does not indicate progress per se, especially when one focuses on homology rather than analogy—look at all the different types of analogous wings and other flying or gliding membranes. Convergence, yes; progressive advancement, no. Bird and bat wings are homologous as vertebrate forelimbs but analogous as wings, for their common ancestor possessed a limb with a radius, humerus, and ulna, but not in a wing form. If all evolutionary roads led to the same destination, this might be construed as evidence for progress, but in fact these roads are at best merely parallel. What might appear from a distance to be a single type of wing turns out upon closer reflection to be several types, each reflecting disparate origins and contrasting ontogenetic/phylogenetic pathways as much as or more than ultimate end points.
Culmination vs. Contingency: How Important Are Stochastic Events in Evolution?
Gould’s Wonderful Life (1989) notion of rewinding and replaying the tape of history, leading to an entirely different present day due to all sorts of chance historical contingencies, provides a valuable starting point for classroom discussions of evolutionary progress and directionality. In Gould’s words (1989), “Life is a copiously branching bush, continually pruned by the grim reaper of evolution, not a ladder of predictable progress.” Yet Conway Morris (2003) argues, contra Gould, that, if the tape of evolution were to be replayed, humans or at least some intelligent biped would in fact be the inescapable end product. Dawkins accepts the role of contingency but argues (1996, 2003, 2004) likewise that evolution is rife with predictably recurring patterns, such as the repeated independent evolution of eyes, electric organs, and so on, and offers as explanation a combination of convergence with constraints, with different lineages facing similar selection pressures and responding with limited solutions. Dawkins (2004) further suggests that watershed events in evolutionary history, including origins of eukaryotic cells, multicellularity, and sexual reproduction, presented major steps in “evolvability” (Wagner 2007), providing raw materials that paved the way for more advanced organisms. Dawkins (2004) is quick to qualify his hesitating support for “progressive evolution” by referring to a “value-free” rather than “value-laden” progress in which change occurs in a direction one might consider desirable—according to some normative value system, as toward humanity (if we are to be considered the pinnacle of life)—which seems reasonable, from our anthropocentric perspective.
Does Natural Selection Lead to Progressively More Adaptive Design?
Arms races, convergence, and constraints on organismal form and function together give at least the appearance of design, which is often taken by those with a limited understanding of evolutionary mechanisms to mean some sort of preordained, intelligently fostered design. In response, Shermer (2006) baldly states that we should “quit tiptoeing around” and admit that design appears in nature, though solely from a step-wise, bottom-up mechanism—what Dennett (1995) refers to as cranes rather than (top-down or divine) skyhooks. Dawkins (1996) introduced the term “designoid” to signify evolutionary design: not designed in an intentional, purposeful sense, but in a non-teleological, evolutionary sense (as with the purpose vs. function heart example). The bottom line is that adaptive evolution might be considered progressive in the sense that it involves a cumulative, ratcheting process, but not in the sense of moving toward a planned outcome or goal (Aristotle’s telos; Williams 1998; Ruse 2000b; Weber 2011). Nonetheless, some continue to argue (e.g., Wright 2001) that life, and humanity in particular, inevitably progresses in this ratcheting fashion due to the steady accumulation of ordered, directed change.
Do Thermodynamics and Energy Flow Lead to Inevitable Evolutionary Progress?
Although evolutionary biologists recognize that life has, since its origins, increased in size and diversity, they do not necessarily accept this as inherently progressive (Chorost 2012). Carroll (2001) argues that because we fixate on complex organisms, the appearance of progress as a necessary “forward march” is illusory. When there is nowhere to go but up, some species will go up. Other scientists (Rubi 2008; Smart 2009; Chaisson 2011) argue that evolution is invariably driven toward progress by certain inescapable processes, among them basic laws of physics. Rubi (2008) claims that order emerges from chaos where there is excess energy—as with Earth’s open system with regard to the sun’s energy (thus countering the inevitable increase in entropy from the second law of thermodynamics)—and therefore complexity arises in living systems because of this favorable energy gradient. Chaisson (2011) argues that energy rate density, a measure of how much energy flows through each gram of a system per second (Chorost 2012), is a universal gauge to quantify the complexity of ordered systems. Analysis of energy rate density, he contends, reveals an unequivocal upward trend in complexity concomitant with the “ascension” of life from microbes to humans and from hunter–gatherers to technological societies. Yet, although we share the planet with many complex organisms, we also witness organisms (including many parasites) that achieve success due to simplicity. Complexity may be a trend, but it is not an inevitability. Naturalistic explanations can be offered for life’s diversity, but they need not imply a forward or upward march.
Just So Stories: Does All Evolution Involve Adaptive Change?
A similar (and similarly big) pitfall for evolutionary educators, related to this problem of directional progress, involves presenting each feature of an organism as an adaptation. This is the unfortunately rife “adaptationist program” criticized by Lewontin and Gould’s “Spandrels of San Marco” paper (1979). Much writing about organisms, not only in texts and secondary literature but in original research papers, holds fast to this paradigm, presenting “just so” stories (ad hoc fallacies) that are in some cases a short step from Kipling’s amusing tales (1902) about how the leopard got its spots or the elephant its trunk. Examples are plentiful in human biology, including the “aquatic ape hypothesis” (hairless humans must have evolved from early hominins that lived in the sea) and ideas about why humans are good runners or why we hold religious beliefs, or about human form, diet, and behavior (Schlinger 1996; Allcock 2001). Darwin’s well-known contemplation (1859) that a bear feeding open-mouthed in a river was potentially on the road to becoming something like a whale could be cited as a just so story—an untested and untestable idea that makes sense and could be true—but examples can be found in modern biology, such as Barash’s analysis of bluebird mating behavior (1976), which Gould (1978) criticized as storytelling.
Educators must make clear that traits may be ecologically “indifferent” (neither beneficial nor harmful) or may “come along for the ride” due to epistasis or genetic linkage. A clear example of linkage involves the wild silver foxes (bred by Soviet biologist Dmitri Belyaev for tameness) which ended up looking much like domesticated dogs with curled tails and floppy ears (Trut 1999). It is possible that traits may have been adaptive at some point but no longer are today. Still other features, like the long, gaudy tails of peacocks, may be advantageous via sexual selection yet maladaptive in the broader arena of natural selection. It is essential for students to learn that selection is not the only agent of evolutionary change, that not all traits are adaptive, and that adaptations are context-dependent. Evolution occurs from genetic drift and gene flow, with migration of individuals into and out of populations, and hence of alleles into and out of gene pools. Mating is often non-random, and of course, many neutral and silent mutations occur at random. Inherited traits are often linked to other traits.
New discoveries in evolutionary developmental biology (evo–devo) have led to great strides in evolutionary thinking, with biologists (Gould 2002; Carroll 2006; Kirschner and Gerhart 2005) explaining the role of “facilitated variation” and the importance of regulatory genes in the origin of key innovations. Kirschner and Gerhart (2005) argue that the “irreducible complexity” argument prominently espoused by intelligent design creationists (Forrest 2008; Shanks and Green 2011) is largely undone by advances in evo–devo, which suggests ways complex organs, such as vertebrate limbs (or the eye that bedeviled Darwin; Buschbeck and Friedrich 2008; Gregory 2008b), may undergo major transformations not from gradual (“progressive”) accumulation of minor mutations in structural genes but instead from minor tweaking of regulatory (e.g., homeobox) genes. Epigenetics is another burgeoning branch of biology that adds richness to the study of phenotype, especially as it suggests ways in which environmental circumstances might lead to genomic imprinting and hence inheritance of traits influenced by long-ago conditions experienced by ancestors (Jirtle and Skinner 2007).
A valuable exposition of this topic, and a cautionary tale about just-so adaptive stories, can be found in Gould’s essay “Of kiwi eggs and the Liberty Bell” (1986), explaining how the disproportionately gigantic eggs of kiwi birds make sense not as an adaptation but as a historical holdover from their much larger ancestors. Gould writes: “This assumption—the easy slide from current function to reason for origin—is, to my mind, the most serious and widespread fallacy of my profession, for it lies embedded in hundreds of conventional tales about pathways of evolution…When you demonstrate that something works well, you have not solved the problem of how, when, or why it arose.” In trying to dispel this Panglossian paradigm, Gould offers a humorous anecdote of his trying to suss out the mythic significance of the words “Pass and Stow” cast onto the Liberty Bell (like the biblical inscription from Leviticus, “Proclaim liberty throughout all the land unto all the inhabitants thereof,” which also appears on the bell), only to find that the words referred simply to the names of the two men who cast the iron bell. Gould noted (1986) that we make this “easy slide” because “we want answers that invoke general laws of nature rather than particular contingencies of history” (Gould 1989) which are not nearly as satisfying to us.
Ladder vs. Bush: Does Evolution Proceed in a Straight Line?
Culmination versus contingency arguments abound in the study of human evolution. These can help students think critically about evolutionary histories depicted as anagenesis or orthogenesis (an inevitable “straight line” of progressive evolution) rather than as chance contingencies, as in Ray Bradbury’s short story “The Sound of Thunder” (1952), in which a man who travels backwards in time to view dinosaurs incidentally swats an insect, then returns to the present only to find that everything has changed. Relethford (2005) gives a similarly clever example from an episode of the original Outer Limits TV show entitled “The Sixth Finger,” in which a time machine with “backward” and “forward” levers allows a man to travel to the future, where he evolves a sixth finger for extra dexterity. The time machine’s “forward” function implies a fixed path for the future, as of the unrolling or unfolding of an already-written scroll. For an alternative view, consider that if our ancestral amphibian had not crawled ashore with a pentadactyl limb, we would have no base-ten (i.e., decimal) mathematics. As Monday morning armchair quarterbacks know, hindsight is 20/20. Postdiction is easy. Prediction is the hard part.
Preadaptation: Planning Ahead?
Preadaptation is a problematic concept related to prediction. Blessed with the gift of foresight, we tend to imagine the future can always be anticipated and that organisms routinely do so, planning progressively. How else to explain the evolution of complex eyes and wings, and the dreaded problem of irreducible complexity (Gregory 2008b)? Yet we now know that changes in regulatory toolkit genes lead to rapid, major shifts in body plans, just as we understand that the functions of structures can change as surely as their forms. Once again, teachers must nimbly navigate a minefield of counterintuitive notions. Did the dinosaurian ancestors of birds use feathers for courtship displays, thermoregulatory insulation, or traps to capture insects before flight evolved? All of these hypotheses involve preadaptation. Insect wings may have arisen from structures used as heat radiators, which pre-adapted some insects for flight capacity (Kingsolver and Koehl 1985). This leads to interesting questions of evolvability and whether some taxa or biological systems enjoy a greater capacity for adaptive evolution (Wagner 2007). This is a fascinating topic, but, to avoid student confusion, it is often better to answer questions about preadaptation with the related (mostly synonymous yet non-teleological) terms exaptation or co-option (McLennan 2008).
Students easily fall into the trap of believing that current structures reflect a progressive change from initial adaptations: We evolved our upright posture so we could reach up and change light bulbs, or so we could punt, pass, and kick a football. Yes, a shoe can serve as a flyswatter, hammer, weapon, percussion instrument, and so on, but those were not its initial function. Could the absence of a tail in humans and apes be linked to sitting and walking upright? That’s an entirely plausible explanation, but that doesn’t mean it is the correct one. Perhaps tail loss was genetically linked to a mutation that gave apes greater shoulder flexibility or larger brains. The flip side, of course, is that just because something is unlikely doesn’t mean it can’t occur. People do win the lottery, even against astonishing odds. Animals do cross oceans on makeshift rafts of palm fronds. Given the extraordinarily vast timescale of evolutionary history, we should not be surprised that unlikely things might occasionally happen.
Aren’t Organisms Optimally Designed?
Another fallacy relating to progressive notions of evolution holds that organisms are perfectly designed. Numerous examples including the panda’s thumb (Gould 1980), male nipples, and countless vestigial organs in humans (Shubin 2008) and other species have yet to dispel this notion. Unfortunately, one might easily presume, given the overwhelming number of species that have gone extinct, that all the extant survivors are success stories that are perfectly adapted to their conditions. A moment’s reflection, however, reminds one of the old joke about the two campers attacked by a bear: The one that survived didn’t need to outrun the bear, but only had to outrun the fellow camper. In the same way, the “winner” of a genuine competition is better than its competitors, but this does not mean it is perfect or close to it. A winner emerges from every golf or tennis tournament, even if none of the competitors is skilled. Ecological concepts of competitive exclusion tell us that the actual (realized) niche of a species will be constrained by competition and that organisms may be forced into poor habitats that would not otherwise be suitable. Plants and animals that live in a harsh desert might “prefer” to live elsewhere but have adapted or otherwise come to live in a place where they can survive the competition.
Students must be reminded that evolution neither crafts perfection nor begins with a fresh blueprint. Form often follows function, but not invariably. Sometimes form follows history, not only in nature but in the human realm as well: The QWERTY keyboard (designed to avoid jammed typebars) is a familiar example, as is the railroad gauge which arose, two millennia later, from the distance between Roman chariot wheels. Other times, neutral form is governed simply by mechanisms of inheritance. We forget that evolution tinkers rather than fashions organisms de novo from a proverbial blank slate. Organisms carry their historical baggage, which explains why humans are prone to bad backs, flat feet, and impacted wisdom teeth, and why we have a vermiform appendix, vestigial ear muscles, and male nipples.
Complexity and Progress: Do All Features of an Organism Evolve at the Same Rate?
A problem that trips up many students is the idea that all features of an organism change at the same rate. This poses a major challenge for thinking about human evolution, as people presume there is a “missing link” somewhere between our own species and our closest living relative that fills a space exactly halfway in between. This claim presumes that all human features (bipedalism, big brain, relative hairlessness, language, and so on) arose at the same time and thus, that there was, at some time, some organism that was “halfway there” along this spectrum. Not only does the fossil record not show any such links (in any lineage), but further reflection reveals that the impression that diverse characteristics of a species appear and evolve at the same rate is unlikely to be supported by paleontology—it is nonsensical.
Instead of this vision of steady evolutionary progress, a more realistic view depicts each species as a mosaic blend of old (primitive) and new (derived) features. As Kardong (2008) points out, the duck-billed platypus, Ornithorhynchus anatinus, is a fine example of this mosaic concept. In form and function alone, there are obvious ways it qualifies as a “primitive mammal” (egg-laying, male mammary glands, weak thermoregulation), but there are just as many other features that make it a highly derived mammal (electroreceptive snout, venom glands). An apparent adverse effect of this view of cumulative, progressive evolutionary change is that students more readily accept evolution of bones and other physical structures rather than of less tangible features like biochemical pathways or behaviors; the latter in particular pose a problem for many people (Werth 2009). In this regard, Dawkins’s idea of the extended phenotype (2004) is especially useful: A beaver dam should be considered an expression of beaver genes.
One of the things we must perpetually bear in mind is that most organisms, or parts of organisms, are complex structures. Is your head for feeding, for breathing, or for sensory input and neural processing—or for all of the above? The head is a highly elaborate functional matrix (Lieberman 2011) with many specific functions that fulfill various biological roles. It must likewise be realized, harking back to the fallacy of optimal design, that many features of organisms represent a functional compromise. The head of the human femur breaks off in many elderly women, but then again the pelvis is a compromise “built” to satisfy competing demands: It would presumably be wider if we were not bipedal, but narrower if we had smaller brains. As it is, the pelvis is just wide (or narrow) enough to allow humans to be both big-brained and bipedal, but it occasionally fails us by leading to broken hips or mothers (and babies) who die while giving birth. Kardong (2008) offers another apt example of a functional compromise, showing the wings of plunging seabirds (boobies, gannets, terns, etc.) as intermediate between the long, narrow wings of soaring gulls and albatrosses and the short, wide wings of swimming penguins. Is this progress?
Mutation on Demand and the “Great Warehouse in the Sky”: Do Organisms Get What They Need or Want?
Consider the following excerpt from a letter (Popielarz 2003) to a newspaper editor, complaining that an article about evolution in bacteria describes no such thing: “I don’t believe the bacteria to which the author is referring are ‘evolving’ into a more complex form such as a guppy; they are adapting to the anti-bacterial agents being used and becoming immune much as a person becomes immune to various illnesses. A dog living outdoors in a cold environment does not evolve a heavy coat of fur and become a bear. Rather, he adapts to the cold by developing a heavy coat.” Surely the letter writer is misinformed about the dog acclimatizing (not adapting) to different seasons, nor does he understand that the bacterial change involves modification from one generation to the next, much as he differs from his parents, and his children, if any, differ from him. Students and others with less background and experience in evolutionary education tend to forget that populations evolve, not individual organisms.
Students also forget just how such changes occur. A fallacy related to progressive “just so stories” involves the mistaken view of mutation on demand. We slide into the trap of thinking organisms choose adaptations they need, or that natural selection simply gives them adaptations. We speciously confuse cause and effect (Mead and Scott 2010b), as by thinking antibiotics cause bacteria to evolve resistance, rather than that some bacteria randomly possess natural resistance, so that survival of these microbes when coupled with the death of non-resistant bacteria leads to a rapid change that looks for all the world (particularly from our teleological perspective) as if the one event directly brought about the other, though this was not of course caused by but instead was an effect of antibiotic use. Organisms cannot change merely by striving or needing, despite the claims of Teilhard de Chardin and Lamarck. To take the classical evolutionary example, giraffes did not attain their height because of “trying” to get taller.
Teleological thinking is undoubtedly commonplace in biology because we intuitively expect living things (and other apparently animate entities, like storms) to act the way we would, with intentions and desires (Mead and Scott 2010a, b). This is commonly manifested in explanations that consciously or subconsciously suggest that biological phenomena involve molecules or cells acting as if they were invigorated and motivated by tiny minds: Ribosomes and RNA molecules busily put together polypeptides like workers on a construction site; hormones, antibodies, and carriers cruise through the bloodstream like morning commuters knowing precisely where they are headed; spermatozoa race toward ova, and pollen tubes eagerly extend toward ovules. The obvious implication is that all these hierarchical levels of biological organization involve subordinate elements of biological systems knowing exactly what they are doing—expressing and fulfilling clear intent. All are on the road to progress. Molecules diffuse through membranes because they “want to” even out concentrations (Douglas Allchin, personal communication).
A problem related to “mutation on demand” is that people often think that, whatever situation an organism faces, it will somehow find a solution (e.g., humans will somehow find a way to evolve to tolerate increasing pollution). You can see manifest dangers in this line of thinking. Not only would there be no imperfect organisms if all species got what they “needed,” but no species would ever go extinct.
A simple plea for teachers to employ clearer language is weak advice—it will be welcome, but what will be more effective is recognizing and calling out mistaken references to intentionality in students’ answers and even in textbook explanations. We get so caught up thinking of nectar as an intentional reward for dutiful pollinators ferrying pollen from one plant to another (where it is “supposed to go”) that we fail to see the situation accurately—not only do we tend not to describe the interaction correctly, but we may not even properly understand the evolutionary and ecological basis of this coevolutionary process, and we may fail to recognize how it arose and how it continues to undergo modification.
Teleological Necessity: Do Organisms Evolve Progressively so as to Fulfill Needs?
Related to the common way of thinking about intentions (desires) of organisms is a presumption that organism X “needs to” or “has to” do Y or Z to survive (as with the letter-writer’s example of the dog and bacteria cited above). Just as we speak of how nucleotide bases in DNA “need to” pair up, we say that organisms have to do this or that to fulfill basic biological functions of feeding, immunity, sensation, and so on. In some students’ minds, just as Pleistocene mammals evolved thicker fur coats or big teeth and claws because they “needed to,” we or our ancestors “had to” evolve big brains, binocular vision, and opposable thumbs. The truth is that organisms do not “need” to defend against pathogens; some coevolve to live with them, and others do not survive. Just as biological systems need not be perfect, they need not persist. We are so eager to speak of harmony in nature (with predators “needed” to regulate prey populations and forest fires “needed” to rejuvenate communities and pave the way for ecological succession) that we fall into the trap of explaining complex systems as if they operated like thinking beings rationally planning their existence. Sadly, such descriptions often follow (in texts and lectures) a caution to avoid teleological or Lamarckian explanations. Long live Lamarck! His discredited ideas are commonly ridiculed in biology class and then unwittingly followed.
Chances are that, if you are reading this paper, you already know this mistaken teleological reasoning well. Part of the problem is that biologists and educators routinely speak in a sort of evolutionary shorthand in which we explain that one thing “led to” another and that taxa are “designed” for X and Y. Teleological issues notwithstanding, this is not a problem if we make clear we are speaking of a “bottom-up” rather than a “top-down,” teleological design (Dennett 1995). Experienced students likely understand that we routinely omit steps of evolutionary explanations for brevity, if not clarity. Nonetheless, beginning biology students will not have the background to appreciate these distinctions. We must clearly spell out all steps in a causal, mechanistic chain (Alles 2005). Dawkins (2004) notes that biologists easily slip into shorthand patterns of speech or writing “because it chimes with the way humans naturally think.” People frequently say that “everything happens for a reason.” This is fine in science so long as we are referring to an initial cause rather than a “final,” teleological cause: an effect, in other words.
Proximate vs. Ultimate: Does Teleonomy Aid in Avoiding the Pitfall of Progress?
Regarding causes and telos, it is difficult for students to remember the key difference between proximate and ultimate causes (Mayr 1993; Shellberg 2001; Ariew 2003; Mead and Scott 2010b). When asked why many organisms engage in sexual reproduction, students promptly respond that it is to generate genetic variation. While this is surely a valid ultimate (evolutionary) cause, it does not explain why we and other organisms enjoy sex. Likewise, we do not sweat to cool off (technically speaking) but rather do so when the hypothalamus triggers glandular secretions on our skin which enable us to cool off. Birds build nests when environmental cues (such as changes in photoperiod) lead to changes in hormone levels. We cannot presume they build nests to have a place in which to lay, hold, incubate, or protect eggs (untestable hypotheses, unlike the first part of this sentence).
A familiar way to avoid such confusion—and perhaps the best way in evolutionary discussions—is to replace teleological explanations (which are goal-directed) with teleonomical explanations, which sound similar but replace final (ultimate) causes with original (proximate) causes and are thus not end-driven. When A→B→C, we must explain B’s occurrence via A as an initial cause, not by C as an eventual effect. This important distinction was first highlighted in 1958 by Pittendrigh, yet it has failed to gain sufficient traction in classrooms. Comparison of the mechanistic, cause-and-effect chain of action in non-living (i.e., unevolved) and living systems, such as thermostats and furnaces vs. the hypothalamus and sweat glands, offers an effective way to distinguish teleonomy from teleology.
Past vs. Present: What Is Past is Prologue, or Past as Past?
The preceding sections highlight another prominent casualty of the standard model of evolutionary change as progress: We fail to see that evolution is an ongoing process and tend rather to view evolution as an unrolled scroll, an event that has fully happened and is now complete. In short, we may see evolutionary processes as a thing of the past rather than present and future; we may see evolutionary pattern as a merely historical phenomenon rather than a continuing, unending (except for taxa that go extinct) prospect. It is thus useful to discuss the evolution of humans and other species in terms not only of what has unfolded but what may yet come to pass. There have been numerous fascinating books and documentaries based upon sound evolutionary reasoning—Dougal Dixon’s After Man (1981) or New Dinosaurs (1988), and the Discovery Channel series The Future Is Wild (2004)—which speculate on what the future might hold as continents and climates continue to change and new species arise.
As anthropologists are fond of pointing out, our anthropocentric view of H. sapiens as the culminating pinnacle of a progressive scheme of evolution is fed by an odd quirk of recent history, namely that since the demise of Neanderthals 30,000 or so years ago, ours is the sole hominin species walking the planet. This has been true for all of our recorded history but was obviously not at all the case for the past several million years, when diverse australopithecines and other hominins lived as contemporaries. Drawing students’ attention to this historical peculiarity is an interesting and effective way to engage them in deeper discussions of evolution, particularly of human evolution (Werth 2009). Just as it is tough for us to think non-teleologically, it is difficult indeed for people whose lives are ruled by the clock and calendar to conceive of the vastness of deep time and hence to relate adaptive microevolution to the macroevolutionary origin of new species.
Even better than these science fiction accounts of what may come to be are long-term experiments run by the Michigan State University lab of Richard Lenski on E. coli (Barrick et al. 2009; Lenski 2011). These hold great promise in aiding students to understand evolutionary processes and their application to current engineering, bioinformatics, and computational problems. Findings by Lenski and other faculty and students at the BEACON Center for the Study of Evolution in Action shed light on the power, process, and pattern of evolutionary change, as do video and computer games (such as Spore, which shows how a microscopic organism might evolve into a complex creature) and program- or module-based digital organism simulators including Creatures, Evolve, Darwinbots, and Primordial Life.
Which Way Is Up? Doesn’t Progress Imply Directionality?
As suggested earlier, our view of progress undoubtedly depends on our perspective. When we view evolution as an intentional process, with organisms “needing” or “trying” to adapt, and with some living things “more highly evolved” than others, our bias shows clearly. Perhaps it is no surprise that people often consider other species (dolphins, for example) as less intelligent because their vocalizations do not reflect the same complexity of symbolic, syntactic language that we possess. On the other hand, we seldom stop to think that on a different scale, with dolphins at the top, they could be said to be (if it made any sense to say it) more highly evolved than we are, given their remarkable abilities, including swimming and leaping, deep diving, and echolocation. Just as the victors get to write the history books, humans write the biology books, and we invariably describe and define a progressive world from our standpoint.
The Advance of Evolutionary Progress: A Straight Line in Behavior, as in Fossil Forms?
If evolution were to reflect an inevitable march of progress, a logical corollary is that modern-day phenomena can be explained by yesterday’s precursors. This is especially common, and unfortunate, in explanations of human behavior. Evolutionary psychology has rightly attracted attention (Geher et al. 2011)—it is only natural that we are curious about our origins and the evolutionary roots of our behavior—but an unwelcome spin-off is the excuse that “my Paleolithic ancestors made me do it.” We are not trapped forever by ancestral habits. Armed with knowledge, we can choose to eat healthy food, even if our appetites crave the fatty, high-calorie food that would have benefited our ancestors greatly. We can choose to exercise, even if people long ago lived such tough lives that they never had a chance to become couch potatoes. We can use birth control or abstain from sex, even if the drive to procreate is one of the strongest in nature. Evolutionary psychology is a potentially valuable field, but to claim that what worked for our ancestors always works for us today, in a very different social environment, is hogwash (Phelan 2001). Many people unfortunately confuse prognosis with prescription, though there is manifestly a huge difference. When a physician says you have condition X, she is not saying this is necessarily a good thing. Likewise, when evolutionary psychologists claim that men seek to have as many sexual partners as possible (so as to better spread their genes), they are not advocating such behavior.
Progress vs. Chance: What’s Random and What Isn’t?
Figuring how directed change can arise from non-directed, chance events is a head-scratcher for many students, especially when they are trying to square it with a progressive view of evolution. Mutations arise at random, but clearly this does not mean that evolutionary change is random. After all, students should recognize that the word “selection” refers, as in human terms, to a chosen option, a beneficial preference (Bardapurkar 2008). Children don’t close their eyes when choosing sides for kickball teams; they don’t “pick” blindly, and neither does selection (natural or artificial, the big difference obviously being that there is no foresight in nature). Herein lies a huge pitfall of evolutionary education: overcoming the mistaken notion that all evolutionary change comes about by chance. To take a familiar example, nicely outlined by Dawkins (2006), equating evolutionary change with a jumbo jet spontaneously assembling out of a junkyard of parts in a windstorm is a ludicrous and wholly unfair accusation. Do plant and animal breeders randomly pick the organisms they will breed, or do they select specimens with physical and behavioral traits they want to amplify in the next generation? This is indeed a cumulative, step-wise (and hence “progressive”) process, as is the construction of a jet airplane, as Darwin carefully explained in using artificial selection to set up his argument for natural selection in Origin of Species (1859), yet it is anything but random. If global temperatures rise or fall, will it be random which species thrive and which suffer hardship? If a population of dogs (not the individual dog of the letter-writer above) is subject to an oncoming ice age, will not those with longer hair enjoy a selective advantage, all other things being equal? Such changes are predictable if we know how the environment will change, but because we cannot, this is not progressive. Some whale ancestors came ashore, and some of their descendants later went back to sea, but neither process involved randomness, nor did they involve progress.
Survival of the Fittest: Doesn’t Progress Mean that the Winners Are the Strongest?
Finally, a major evolutionary misconceptions stems from Herbert Spencer’s unfortunate phrase “survival of the fittest,” which too few people recognize as referring to “fitting” the environment and too many people take to mean the biggest or strongest (and thence to “Social Darwinism”). Allchin’s suggested “amplification of the aptest” (2007) is a worthy candidate to replace “survival of the fittest,” reflecting the fact that even individuals that are less fit can survive, though their genes will likely not be as well represented in the succeeding generation. Regrettably, stereotypical notions of evolution as competition (with a clear winner and loser) further foster belief in evolution as progress and lead one to wonder how descent with modification can lead to “inferior” species that persist today (Schwenk 2002), hence the perennial query, “If we evolved from chimpanzees, why are they still here?” This “chimp fallacy” is fostered by old-school ladder-like (rather than bush-like) evolutionary diagrams (Mead 2009; Kumala 2010; Meikle and Scott 2010). Just as you and your cousins share descent from a common grandparent, so too species share descent from common ancestors. There is a fundamental difference between lineal and collateral relatives, but even direct lineages need not involve progress.
